Make sure retrieving _USE_VFORK can't fail

[nezgrath]

Description

This pull request refines the logic for disabling vfork and posix_spawn in subprocess management within the PrepareSubprocess class. The changes improve platform checks, ensure safe attribute handling, and prevent unnecessary modifications on unsupported platforms.

Improvements to subprocess management logic:

• The logic now correctly checks platform compatibility and only disables vfork and posix_spawn when appropriate, preventing changes on unsupported platforms.
• Added safe attribute checks using hasattr before modifying subprocess._USE_VFORK and subprocess._USE_POSIX_SPAWN, reducing the risk of errors if these attributes are missing.
• Original values of subprocess._USE_VFORK and subprocess._USE_POSIX_SPAWN are now stored and only restored if they were previously set, ensuring proper cleanup.

Checklist

• I have tested these changes locally.
• I have added unit tests (if appropriate).
• I have added necessary documentation or updated existing documentation.
• I have linked the issue(s) addressed by this PR if any.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.11%. Comparing base (cd04de2) to head (7512448).

Files with missing lines	Patch %	Lines
src/ansys/hps/data_transfer/client/binary.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
- Coverage   85.17%   85.11%   -0.06%     
==========================================
  Files          17       17              
  Lines        1551     1559       +8     
==========================================
+ Hits         1321     1327       +6     
- Misses        230      232       +2     

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